Feedback amplifier



Patented Sept. 30,` 1947 FEEDBACK AMPLIFIER George T. Royden, SouthOrange, N. J., assignor to Federal Telephone and Radio Corporation, acorporation of Delaware Application June 2.0, 1942, Serial No. 447,851

15 Claims.

This invention relates to amplifiers and particularly to ampliers havingfeedback connections for controlling the gain. The object f thisinvention is to provide an efficient feedback and gain control circuitwhich will reduce the gain `of the amplifier during Ythe duration of anunusually high amplitude signal, so as to prevent` distortion.

In accordance with my invention, I reduce the gain of an amplifier whenan especially high amplitude signal is momentarily present, by means ofa reverse feedback connection between the output and the input of theamplifier so connected that when the signal strength is increased abovea certain level, the gain of the amplifier is decreased. I accomplishthis by placing in the feedback circuit a shunt comprising a variablev utube whose impedance is dependent on a D. C. control voltage derivedfrom the amplifier output. 'I'he invention will be better understoodfrom the following description and the accompanying drawing of whichFig. 1 illustrates a system embodying the invention;

Fig. 2 illustrates a system using a variation in accordance with theinvention;

Fig. 3 shows another variation of the invention; and

Fig. 4 illustrates another modification.

Referring to Fig. 1 there is shown a vacuum tube amplifier comprising aninput transformer I adapted to have applied to it electric signals ofvarying amplitude. The rheostat 2 connected across the transformeroutput may be a manually operable gain adjustment device. Tube 3 is anamplifier receiving energy from the input transformer, the tube 3 havingconnected with it a cathode resistor Il, a screen resistor 5 with abypass condenser 6, a plate resistor l, and a series coupling capacitor8 to the control grid of the succeeding amplifier tube 9. Tube 9similarly has connected with it a grid resistor Ill, cathode resistor IIwith by-pass condenser I2, a plate resistor I3 and a series couplingcapacitor I4 leading to the control grid of a third amplifier tube I5.Tube I5 likewise has connected with it a cathode resistor I6, by-passcondenser I'I, grid resistor I8, and has an output transformer I9.

The anode current for all tubes is supplied .by a power supply unit 24which takes power from the A. C. lines, rectiiies it, and sends therectied current through the series connected bleeder resistors 25, 26and 2l. Anode current is supplied from the bleeder line to the inputstage 3 through resistor 2D by-passed by capacitor 2| and to theintermediate stage 9 through resistor I3 and to the output stage throughresistor 22 by-passed by capacitor 23. The cathode heater leads marked Hmay be heated by corresponding leads I-l from the power supply.

The reverse feedback and gain control circuit is provided as follows. Anauxiliary winding 49 on the output transformer I9 has connected one sideto the anode of a diode rectier 28 whose cathode is biased positive byconnection to a point on the bleeder resistor 26, and has its other sideconnected through a resistor 38 to a more negative point on the powersupply bleeder circuit. Resistor 38 is shunted by a capacitor 29 and isconnected to the control grid of a variable .a tube 39, the cathode andsuppressor electrodes of which are connected to a point on bleederresistor 2l more positive than the grid, the plate of which is suppliedthrough resistor 35 and the screen is supplied through resistor 3S whichis by-passed by capacitor 3l. The variable p. tube and resistor 35constitutes the shunt arm of a T network, one series arm of which isconstituted by a resistor 3l and blocking capacitor 32 and the otherseries arm of Iwhich is constituted by a resistor 33 and blockingcapacitor 34. These series arms are comprised in the feedback path fromthe secondary coil 39 of output transformer I9 to the cathode of theinput stage 3, the polarity being such that the feedback is negative.Thus the circuit for the feedback current is traced from ground, throughone part of the output transformer secondary, through series elements3|, 32, 34 and 33, through the condenser 4I and resistance 42 connectedbetween.

the cathode and grid of stage 3, and back to ground. At the same time apredetermined amount of feedback current is shunted to ground throughresistor 35 and capacitor 43, also through the normal impedance of tube3B which may be adjusted lby variable resistor 2l to the desiredoperating point. The combination of ycapacitor III with resistors 4 'and42 permit control of the phase of the feedback energy.

When a signal which'is too strong is passed through the amplifier ortries to pass through the amplifier, a portion of the output isrectified in diode 2B and charges capacitor 29 negatively with respectto ground, thereby imposing a more negative bias on the control grid ofthe variable c tube 39, which increases its internal impedance so thatthe shunt arm impedance of the T network becomes large and a relativelylarge part of the negative feedback energy reaches the input ofamplifier tube 3, and thus tends to decrease the strength of the signalpassing through it. When the strong signal has passed, the charge oncapacitor 29 occasioned by the rectied current slowly leaks oli throughresistor 38 thus restoring the system to normal after the passage of theexcessive peak signal. In this Way temporary peak signal strengths arenot permitted to overload and distort the amplifier' The invention isnotv limited to any particular values of circuit constants, and thesemay be so chosen and the cathode bias of tube 30 so ad justed that theinternal impedance of tube3 is normally small in comparison with theimpedance of the series arms of the T network; hence by-passing most ofthe energy returning from the amplifier output to the amplifier inputand thereby providing a relatively small amount of feedback.

The invention is not necessarily limited to they particular number ofstages shown in Fig. 1 as it may be vapplied .to amplifiers havingeitherv more or less than three stages. Furthermore, variations in thereverse feedback and gain control circuit may be made within the scopeof the invention. For example, the arrangement may be applied to onlysome of the stages in a multiple stage amplifier.

Another embodiment of the invention is illustrated in Fig. 2 which isthe same as that shown in Fig. l except that the resistor 3 I isconnected to the plate of tube I5 instead of to winding 39, resistor 33is connected to the cathode of tube 9 instead of to tube 3, andcapacitors l2 and 4l and 4resistor 42, shown in Fig. l, are omitted fromFig. 2.

A further variation is illustrated in Fig. 3 which illustrates part ofthe arrangement shown in Fig. 2. The arrangement in Fig. 3 is the sameas that in Fig. 2 except that in Fig. 3 tube 3 is a variable a tube andits grid potential is made subject to automatic gain control byconnecting the lower junction of ythe secondary of transformer l andrheostat 2 to the junction of capacitor 29 and coil 40, instead of toground as in Fig. 2.

Another modiiication is illustrated in Fig. 4 which is similar to Fig.31 except that in Fig. 4 there is added an additional diode typerectifier tube ed having its anode connected with the lower end ofresistor 2 and its cathode connected through an adjustable resistor 41to the anode of the output tube l5. This provides automatic control ofthe gain of tube 3 dependent on the average signal level. The signalenergy i-s rectied-in tube All and stored in capacitor 45, the voltageacross this capacitor being applied to the grid of the variable 1i tube3. Resistor 4B dissipates the energy stored in capacitor 45, thustending to increase the gain of tube 3. Adjustable resistor 47 providesmeans for adjusting the rate of decrease in the gain of tube 3. Theamplier will therefore automatically regulate its gain to maintainV asuitable output level. This gain control will usually be relatively slowacting, but the control of the reverse feedback by signals in excess ofa predetermined level will be quick acting in order to avoid excessivepeak levels.

By the use of lthis combination, the rectifier 44 and variable ii tube 3may be adjusted relative to each other to provide a higher gain than ifrectifier 23 and its associated variable impedance element were notpresent. The reason for this is that the operation of rectifier 28 andthe negative feedback connection is to remove excessive signal peaks,and this allows the amplier to be operated at a highersignal level thanotherwise.

In addition, other modifications of the invention are possible as maybecome apparent to those skilled in the art.

The reversed feedback arrangement according to this invention has agreat advantage over prior known systems in that in the present systemthe reversed feedback is most eective when it is most'needed, which iswhen the signal amplitude is greatest. Usually the greatest distortionoccurs when the signal amplitude is highest, and this amplifierarrangement reduces the gain of the amplifier by increasing the reversedfeedback thereby making use of the property of reversed feedbackamplifiers to decrease distortion in proportion to the amount ofreversed feedback.

What is claimed is:

1. In combination, an amplier having an output and an input, a negativefeedback connection between the output and the input connected in thephase which tends to oppose the phase of the signal in the amplifier, avariable impedance shunted across said feedback connection and having afdeterminable impedance-current characteristic, and means for increasingcontinuously non-linearly the relative impedance of said variableimpedance in response to an increase in signal strength.

2. In combination, an amplier having an output and an input, a negativefeedback connection extending between the output and the input in thephase which opposes the signal impressed on the input, a variable Vacuumtube shunted across said feedback connection and means for applying onsaid tube a biasing voltage derived from said amplifier output so thatthe impedance of said tube increases with increased signal .strength inthe amplifier.

3. A combination according to claim 2 in which the means for applyingthe biasing potential comprises a rectifier which produces a rectifiedvoltage component dependent on said amplifier output and applies saidcomponent to a; control electrode of said variable ,i tube.

4. A combination according to claim 2 in which said feedbacky connectioncontains an impedance in series between said output and said input.

5. In combination, an amplifier having an output and an input, anegative feedback connection between the output and the input in thephase which opposes the signal strength impressed on sald input,impedance elements connected in series in said connection, a variable ,itube in shunt with said connection and a rectifier which receives energyfrom said output and applies a rectified voltage'to a control element ofsaid tube, said tube having anormal impedance which is small incomparison with the series impedance in said connection arranged to passa principal portion of the feedback current lthrough the tube. and a,relatively high impedance at rectied voltages produced during outputsurges, arranged to maintain in the negative feedback connection aportion of the feedback current suiiicient for substantial balancing ofsaid surges.

6. In combination, an amplifier having an output and an input, anegative feedback connection containing an impedance between the outputand the input, a second impedance containing a variable i tube in shuntwith the rst mentioned impedance, a condenser across the input of saidvariable [i tube, a rectifier connected to produce a rectified currentin response to said amplifier output, said rectifier having a circuitlwhich causes saidcondenser to charge when a voltage surge is presentedin said amplifier, in the polarity which increases the impedance of saidvariable ,i tube.

7. In combination with a vacuum tube amplifier, a controlling meanscomprising a negative feedback circuit connected from the output to theinput of the amplifier said feedback circuit comprising a T networkhaving series and shunt impedances, the shunt impedance comprising avariable p. tube having a capacitor connected across its input, arectifier connected to the output of said amplifier to produce therefroma rectiiied current, and means for charging said condenser with saidcurrent in the polarity which increases the impedance of said variable,u tube when a voltage surge is present.

8. In combination with a vacuum tube amplifier covering a plurality ofstages, an output transformer at one stage and a cathode at the input ofa preceding stage, a controlling means comprising a negative feedbackpath connected from the secondary of said output transformer to saidcathode, said feedback path comprising a network having series and shuntimpedances, the shunt impedance comprising a variable pi tube having acapacitor connected across its input, and a rectifier coupled to theoutput of said amplifier to produce therefrom a rectified current, a,circuit through said rectifier and said condenser for charging saidcondenser in the polarity which increases said impedance of saidvariable p. tube when a voltage surge is present.

9. In combination with a vacuum tube amplifier having a plurality ofstages including an input control electrode and an output electrode, acontrolling means comprising a negative feedback path connected fromsaid output electrode to said control electrode, said feedback pathcomprising a network having series and shunt impedances, the shuntimpedance comprising a variable c tube having a capacitor connectedacross its input, a rectier coupled with the output of said amplifier toproduce therefrom a rectied current, and a circuit through saidrectifier and said condenser for charging said condenser with saidcurrent in the polarity which increases the impedance of said variableii tube when a voltage surge is present.

10. In combination with a vacuum tube amplifier having a plurality ofstages, an output electrode at one stage and a cathode at a precedingstage, a controlling means comprising a negative feedback path connectedfrom said output electrode to said cathode, said path comprising anetwork having series and shunt impedances, the shunt impedancecomprising a variable p. tube and a capacitor connected across itsinput, a rectifier connected to the output of said amplier to producetherefrom a rectified current, a circuit through said rectifier and saidcondenser for charging said condenser in the polarity which increasessaid impedance of said variable ii tube When a voltage surge is present,and a connection from the input electrode of said preceding stage to theinput side of said condenser.

11. Apparatus according to claim in which said preceding stage comprisesa variable i tube.

12. In combination, an amplifier having input and output circuits and avariable gain stage, a negative feedback connection having a variableimpedance element connected from the output of a stage to the input of apreceding stage, a rectifier responsive to the signal at the amplifier,and a connection for applying the output of said rectifier to controlthe gain of said variable gain stage and also the impedance of saidvariable impedance element so as to increase the negative feedback anddecrease the gain of said stage when the rectifier output increases.

13. In combination, an amplifier, a negative feedback connection fromthe point of the amplifier to a preceding point of the amplifier, saidconnection containing a variable impedance element, a rectifierresponsive to a signal at the amplifier, and means for applying theoutput of said rectifier to said Variable impedance element s0 that whenthe signal strength increases the negative feedback increases, and asecond rectifier responsive to a signal at the amplifier, the output ofsaid second rectifier being connected to control the gain of saidamplifier in response to said signal.

14. Apparatus according to claim 13 in which said amplifier contains aVariable c tube in a stage thereof and said second rectier controls thegain of said variable ,u tube.

15. In combination, an amplifier having a variable gain stage, anegative feedback connection from a point of the amplier to a precedingpoint of the amplifier, said connection containing a variable impedanceelement, a rectier responsive to a signal at the amplifier, and aconnection for applying the output of said rectifier to said variableimpedance element so that when the signal strength increases thenegative feedback increases, and a second rectifier responsive to asignal at the amplifier, a capacity connected to said second rectier tostore output energy from said second rectifier, a resistor connected todissipate energy stored in said capacitor, and a connection from saidcapacitor to a control element of said variable gain stage.

GEORGE T. ROYDEN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,290,084 Albright July 14, 19422,129,074 Thierbach Sept. 6, 1938 1,986,644 Fster Jari. 1, 19352,096,793 Doba Oct. 26, 1937 2,151,829 Barton Mar. 28, 1939 2,222,933Blumlein Nov. 26, 1940 2,269,408 Kinsburg Jan. 6, 1942 2,343,207Schrader et al Feb. 29, 1944 2,281,644 Weathers May 5, 1942 1,959,062Place May 15, 1934

